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      12-07-2007, 04:56 AM   #185
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Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

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Originally Posted by bruce.augenstein@comcast. View Post
OK, now that the smoke has cleared and everyone has cooled off a bit, I thought I'd address these simulations.
Well, Bruce, what continues to be seriously flawed is your interpretation and understanding of the basics of simulation, accuracy, and random vs. systematic errors. All of the numbers you have posted (your personal runs, etc.) are basically “noise” clouding the basic issue here.

By the way I’ll never let you forget that you are the one so foolish to claim here on this very form that you “contributed” to a similar simulation software tool capable of validation with actual tests to within HUNDREDTHS OF A SECOND. Let me quote you, just to remind,

I helped design a quarter-mile simulation tool a number of years ago, called "ShiftMaster", so I have a little knowledge of the topic. That tool was accurate to within around a hundredth anywhere during the quarter mile. I have no knowledge of CarTest, but assume from your rantings that it's pretty good.
I don’t want to get into a detailed case by case validation of CarTest. That really is not the way to validate it anyway. I should do a detailed statistical analysis with means and standard deviations of the test data (for many vehicles) along with convergence and Monte-Carlo analysis (parameter variation, basically) on the simulation side and then I could rigorously establish the accuracy of the tool and the accuracy would be metric dependent (i.e. a different accuracy depending on the test). Not wanting to get so formal (and waste so much time!) I have resorted to a process of informed inductive reasoning; simply comparing it to a variety of cars using a variety of metrics combined with a good understanding of the test measurement process, simulation in general, statistics, error analysis and physics to show me that it is “reasonable” (I’ll define reasonable soon…). It has shown quite reasonable results for key stats such as 0-60, 0-100, 0-150 (all mph) as well as 1/4 mi time and traps. I have also done some work on in gear speed to speed times and some rolling start tests allowing gear changes. Again results have compared reasonably well with tests. I have not performed validations nor claimed accuracy outside of this fairly narrow domain of metrics (for instance speed gained in last 1/8th mi of the 1/4 mi). I do not doubt that some predictions from the software are worse than others. That is the basics of simulation, numerics and physics, it is generally harder to predict the derivative (and since your not really a math guy I mean formal mathematical derivate or slope of something) rather than the something itself. So back on point what do I mean by “reasonable”? I mean within a few tenths here or there in the lower speeds, maybe a second or so on the fastest ones and generally a few tenths and always less than 5 mph for the 1/4 trap speed (often much better than that on trap). Why are these numbers “reasonable”? Simply because they are typically within the average numbers reported from tests plus or minus the variation in the reported test numbers. This is the hallmark of an acceptable/reasonable simulation. Each test is attempted to be controlled but in fact is fairly substantially uncontrolled, major contributing differences include driver, technique, temperature, surface quality, tires (type and inflation) age of vehicle, natural differences vehicle to vehicle, etc. If you want to see this “in action” simply have a look at the thread here called “More performance figures”. It shows this real world variability for the E92 M3 as well as for a bunch of its competitors. So the result of all this is a fairly large variation in test, none for simulation. The simulation is deterministic; same inputs always gives the same outputs, exactly.

This all comes right back to your infamous quote above. There is simply no way to make a simulation tool that matches a widely variable real world test “perfectly” (with my basic definition of perfect here being +/- 1/100th second – right from the horses mouth).

Now drag racing IS significantly more consistent than magazine tests. This is their business – consistency. How does simulation account for this? You simply need more accurate physical tests to determine the best set of input parameters. As well you will likely need an improved simulation tool; specifically an improved friction/tire spinning/tire growth model than available in CarTest. However, the same basic numerical integration scheme based on torque and gears and drag will still work beautifully. Once these and oher SYSTEMATIC ERRORS are reduced guess what? One will be able to get a nice match between these very consistent tests and a simulation. As good as you might get it though your previous claim about 1/100th second is still a farse and a pipe dream.

What else have I said about simulation? Simulation is almost always better at relative predictions compared to absolutes. For instance what change would I see if my M3 had 30 more hp or what is the difference between the M3 and RS4 in a given performance contest.

The whole new car vs. old car thing is so simple I don’t even feel it warrants a reply but here you go. A broken in car will typically produce more power than a new car. If you can measure that effect on an accurate ENGINE dyno and put the results into the simulation guess what – you could predict these differences, again, “reasonably”.

I also disagree with your closing PS. A high redline and gear ratios absolutely have a large impact on vehicle performance both directly and indirectly. Sure, as I have stated and agreed with many on the point, time and time again, the key figure for either track or strip performance is simply power to weight. But gearing and redline matter as well. Gears provide torque multiplication and they do so in all gears at all times. Larger ratios either gear or final = more torque multiplication = more acceleration, period (with the caveat that indeed the wheels are not spinning). Of course there is a limit to this, there is no such thing as a free lunch, you can’t simply raise and raise the gear ratio endlessly, traction is a limiting factor at lower speeds as well is keeping the engine in its best operating rpm band where is produces high torque lastly there is the limit of having such large ratios that it requires too much shifting. A high redline allows one to stay in a gear longer and maximize time spent accelerating instead of shifting (slowing down) and shifting to the next gear where acceleration will be markedly less. Would you disagree that raising the redline on a car, all other things equal (and assuming it still produces reasonable torque in the increased band) it will make the car accelerate better through the gears? The indirect benefits of a high redline and numerically larger gears is more evident on the track than the strip. Vehicles designed as such are typically low torque, high power, high redline, and light/low intertia drivetrain. All of these add up to a car that can not only accelerate but one that can handle.